This invention relates to earth contact monitoring devices for polyphase networks, and more specifically to devices which provide an indication when one or more of the phases in a polyphase network are in electrical contact with the earth.
Known methods and devices for monitoring earth contact of polyphase electrical networks have required that the network be energized with auxiliary potential having determined frequency and wave shape. Generally, the characteristics of the auxiliary voltages are monitored and evaluated to determine whether any of the phases has made contact with the earth. Although existing earth contact monitoring systems such as those described in German references DE-OS No. 24 58 530, and DE-AS No. 25 45 315 have accurately detected earth contact by one or more of the conductors in a polyphase system, the required specific auxiliary voltages must be produced by separate signal generators which produce signals of predetermined characteristics. Thus, the use of auxiliary signal generators increases the cost and complexity of the earth contact monitoring system. In addition, the complexity of such systems renders them susceptible to failure.
Accordingly, it is an object of this invention to provide an earth contact monitoring device for a polyphase electrical network which is inexpensive and simple to use.
It is a further object of this invention to provide an earth contact monitoring device which does not require auxiliary voltages or signal generators.